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Nuclear weapon design

The first nuclear weapons, though large,
cumbersome and inefficient, provided the basic design building blocks of all
future weapons. Here the Gadget device is prepared for the first nuclear test: Trinity.

Nuclear weapon designs are physical, chemical, and engineering arrangements
that cause the physics package[1] of a
nuclear weapon to detonate. There are three basic
design types. In all three, the explosive energy is derived primarily from
nuclear fission, not fusion.

Pure fission weapons were the first nuclear weapons built and the only type
ever used in warfare. The active material is fissile uranium (U-235)
or plutonium (Pu-239), explosively assembled into a chain-reactingcritical
mass by one of two methods:

Gun assembly, in which one piece of fissile
uranium is fired down a gun barrel to a fissile uranium target at the end of
the barrel (plutonium can be used in this design, but it has proven to be
impractical), or

Implosion, in which a fissile mass of either material (U-235, Pu-239, or a
combination) is surrounded by high explosives that compress the mass, resulting
in criticality.

Fusion-boosted
fission weapons improve on the implosion design. The high temperature and
pressure environment at the center of an exploding fission weapon compresses
and heats a mixture of tritium and deuterium gas (heavy isotopes of hydrogen). The
hydrogen fuses to form helium and free neutrons. The energy release from fusion
reactions is relatively negligible, but each neutron starts a new fission chain
reaction, greatly reducing the amount of fissile material that would otherwise
be wasted. Boosting can more than double the weapon's fission energy
release.

Two-stage thermonuclear weapons are essentially a
daisy
chain of fusion-boosted fission weapons, with only two daisies, or stages,
in the chain. The second stage, called the "secondary," is imploded by x-ray
energy from the first stage, called the "primary." This radiation implosion is
much more effective than the high-explosive implosion of the primary.
Consequently, the secondary can be many times more powerful than the primary,
without being bigger. The secondary could be designed to maximize fusion energy
release, but in most designs fusion is employed only to drive or enhance
fission, as it is in the primary. More stages could be added, but the result
would be a multi-megaton weapon too powerful to be useful. (The United States
briefly deployed a three-stage 25-megaton bomb, the B41, starting in 1961. Also in 1961, the Soviet Union
tested, but did not deploy, a three-stage 50-megaton device, Tsar
Bomba.)

Pure fission weapons are always the first type to be built by a nation
state, and, if such a thing should happen, would be the type built by a
non-state terrorist organization,[2]. Large
industrial states with well-developed nuclear arsenals have two-stage
thermonuclear weapons, which are the most compact, scalable, and cost effective
option once the necessary industrial infrastructure is built.

All innovations in nuclear weapon design originated in the United
States;[3] the following descriptions
feature U.S. designs.

In early news accounts, pure fission weapons were called atomic bombs or
A-bombs, a misnomer since the energy comes only from the nucleus of the atom.
Weapons involving fusion were called hydrogen bombs or H-bombs, also a misnomer
since their destructive energy comes mostly from fission. Insiders favored the
terms nuclear and thermonuclear, respectively.

The term thermonuclear refers to the high temperatures required to initiate
fusion. It ignores the equally important factor of pressure, which was
considered secret at the time the term became current. Many nuclear weapon
terms are similarly inaccurate because of their origin in a classified
environment. Some are nonsense code words such as "alarm clock" (see below).

Nuclear reactions

Nuclear fission splits the heaviest of atoms to form lighter atoms. Nuclear
fusion bonds together the lightest atoms to form heavier atoms. Both reactions
generate roughly a million times more energy than comparable chemical
reactions, making nuclear bombs a million times more powerful than non-nuclear
bombs.

In some ways, fission and fusion are opposite and complementary reactions,
but the particulars are unique for each. To understand how nuclear weapons are
designed, it is useful to know the important similarities and differences
between fission and fusion. The following explanation uses rounded numbers and
approximations.[4]

Fission

Fission can be self-sustaining because fission produces more neutrons of the
speed required to cause new fissions. When a free neutron hits the nucleus of a
fissionable atom like uranium-235 ( 235U), the uranium splits into two smaller
atoms called fission fragments, plus more neutrons.

The uranium atom can split any one of dozens of different ways, as long as
the atomic weights add up to 236 (uranium plus the extra neutron). The
following equation shows one possible split, namely into strontium-95 ( 95Sr),
xenon-139 ( 139Xe), and two neutrons (n), plus energy:[5]

The immediate energy release per atom is 180 million electron volts (MeV), i.e. 74 TJ/kg, of which 90% is
kinetic energy (or motion) of the fission fragments, flying away from each
other mutually repelled by the positive charge of their protons (38 for
strontium, 54 for xenon). Thus their initial kinetic energy is 67 TJ/kg, hence
their initial speed is 12,000 kilometers per second, but their high electric
charge causes many inelastic collisions with nearby nuclei. The fragments
remain trapped inside the bomb's uranium pit until their motion is converted
into x-ray heat, a process which takes about a millionth of a second (a
microsecond).

This x-ray energy produces the blast and fire which are the purpose of a
nuclear explosion.

After the fission products slow down, they remain radioactive. Being new
elements with too many neutrons, they eventually become stable by means of beta
decay, converting neutrons into protons by throwing off electrons and gamma
rays. Each fission product nucleus decays between one and six times, average
three times, producing radioactive elements with half-lives up to 200,000
years.[6] In reactors, these products are
the nuclear waste in spent fuel. In bombs, they become radioactive fallout,
both local and global.

Meanwhile, inside the exploding bomb, the free neutrons released by fission
strike nearby U-235 nuclei causing them to fission in an exponentially growing
chain reaction (1, 2, 4, 8, 16, etc.). Starting from one, the number of
fissions can theoretically double a hundred times in a microsecond, which could
consume all uranium up to hundreds of tons by the hundredth link in the chain.
In practice, bombs do not contain that much uranium, and, anyway, just a few
kilograms undergo fission before the uranium blows itself apart.

Holding an exploding bomb together is the greatest challenge of fission
weapon design. The heat of fission rapidly expands the uranium pit, spreading
apart the target nuclei and making space for the neutrons to escape without
being captured. The chain reaction stops.

Materials which can sustain a chain reaction are called fissile. The two
fissile materials used in nuclear weapons are: U-235, also known as highly enriched uranium
(HEU), oralloy (Oy) meaning Oak Ridge Alloy, or 25 (the last digits of the
atomic number, which is 92 for uranium, and the atomic weight, here 235,
respectively); and Pu-239, also known as plutonium, or 49 (from 94 and
239).

Uranium's most common isotope, U-238, is fissionable but not fissile. Its
aliases include natural or unenriched uranium, depleted uranium (DU), tubealloy (Tu), and 28. It cannot
sustain a chain reaction, because its own fission neutrons are not powerful
enough to cause more U-238 fission. However, the neutrons released by fusion
will fission U-238. This reaction produces most of the energy in a typical
two-stage thermonuclear weapon.

Fusion

Fusion cannot be self-sustaining because it does not produce the heat and
pressure necessary for more fusion. It produces neutrons which run away with
the energy. In weapons, the most important fusion reaction is called the D-T
reaction. Using the heat and pressure of fission, hydrogen-2, or deuterium (
2D), fuses with hydrogen-3, or tritium ( 3T), to form helium-4 ( 4He) plus one
neutron (n) and energy:[7]

Notice that the total energy output, 17.6 MeV, is one tenth of that with
fission, but the ingredients are almost one-fiftieth as massive, so the energy
output per kilo is greater. However, in this fusion reaction 80% of the energy,
or 14 MeV, is in the motion of the neutron which, having no electric charge and
being almost as massive as the hydrogen nuclei that created it, can escape the
scene without leaving its energy behind to help sustain the reaction – or to
generate x-rays for blast and fire.

The only practical way to capture most of the fusion energy is to trap the
neutrons inside a massive bottle of heavy material such as lead, uranium, or
plutonium. If the 14 MeV neutron is captured by uranium (either type: 235 or
238) or plutonium, the result is fission and the release of 180 MeV of fission
energy, which will produce the heat and pressure necessary to sustain fusion,
in addition to multiplying the energy output tenfold.

Fission is thus necessary to start fusion, to sustain fusion, and to
optimize the extraction of useful energy from fusion (by making more fission).
In the case of a neutron bomb, see below, the last-mentioned does not apply
since the escape of neutrons is the objective.

Tritium production

A third important nuclear reaction is the one that creates tritium,
essential to the type of fusion used in weapons and, incidentally, the most
expensive ingredient in any nuclear weapon. Tritium, or hydrogen-3, is made by
bombarding lithium-6 ( 6Li) with a neutron (n) to
produce helium-4 ( 4He) plus tritium ( 3T) and energy:[7]

A nuclear reactor is necessary to provide the neutrons. The industrial-scale
conversion of lithium-6 to tritium is very similar to the conversion of
uranium-238 into plutonium-239. In both cases the feed material is placed
inside a nuclear reactor and removed for processing after a period of time. In
the 1950s, when reactor capacity was limited, for the production of every atom
of tritium the production of an atom of plutonium had to be dispensed with.

The fission of one plutonium atom releases ten times more total energy than
the fusion of one tritium atom, and it generates fifty times more blast and
fire. For this reason, tritium is included in nuclear weapon components only
when it causes more fission than its production sacrifices, namely in the case
of fusion-boosted fission.

However, an exploding nuclear bomb is a nuclear reactor. The above reaction
can take place simultaneously throughout the secondary of a two-stage
thermonuclear weapon, producing tritium in place as the device explodes.

Of the three basic types of nuclear weapon, the first, pure fission, uses
the first of the three nuclear reactions above. The second, fusion-boosted
fission, uses the first two. The third, two-stage thermonuclear, uses all
three.

Pure fission weapons

The first task of a nuclear weapon design is to rapidly assemble, at the
time of detonation, more than one critical
mass of fissile uranium or plutonium. A critical mass is one in which the
percentage of fission-produced neutrons which are captured and cause more
fission is large enough to perpetuate the fission and prevent it from dying
out.

Once the critical mass is assembled, at maximum density, a burst of neutrons
is supplied to start as many chain reactions as possible. Early weapons used an
"urchin" inside the pit containing non-touching interior surfaces of polonium-210
and beryllium. Implosion of the pit crushed the urchin, bringing
the two metals in contact to produce free neutrons. In modern weapons, the
neutron generator is a high-voltage vacuum tube containing a particle accelerator which bombards a
deuterium/tritium-metal hydride target with deuterium and tritium ions. The resulting
small-scale fusion produces neutrons at a protected
location outside the physics package, from which they penetrate the pit. This
method allows better control of the timing of chain reaction initiation.

The critical mass of an uncompressed sphere of bare metal is 110 lb (50 kg)
for uranium-235 and 35 lb (16 kg) for delta-phase plutonium-239. In practical
applications, the amount of material required for critical mass is modified by
shape, purity, density, and the proximity to neutron-reflecting material, all of which affect the
escape or capture of neutrons.

To avoid a chain reaction during handling, the fissile material in the
weapon must be sub-critical before detonation. It may consist of one or more
components containing less than one uncompressed critical mass each. A thin
hollow shell can have more than the bare-sphere critical mass, as can a
cylinder, which can be arbitrarily long without ever reaching critical
mass.

A tamper is an optional layer of dense material surrounding the fissile
material. Due to its inertia it delays the expansion of the reacting material,
increasing the efficiency of the weapon. Often the same layer serves both as
tamper and as neutron reflector.

Gun-type assembly weapon

Little Boy, the Hiroshima bomb, used 140 lb (64 kg) of Uranium with an
average enrichment of around 80%, or 112 lb (51 kg) of U-235, just about the
bare-metal critical mass. (See Little Boy article for a detailed drawing.) When assembled
inside its tamper/reflector of tungsten carbide, the 140 lb was more than twice
critical mass. Before detonation, it was separated into two sub-critical
pieces, one of which was later fired down a gun barrel at the other. About 1%
of the uranium underwent fission; the remainder, representing 98% of the entire
wartime output of the giant factories at Oak Ridge, scattered
uselessly.[citation needed]

The inefficiency was caused by the speed with which the uncompressed
fissioning uranium expanded and became sub-critical by virtue of decreased
density. Despite its inefficiency, this design, because of its shape, was
adapted for use in small-diameter, cylindrical artillery shells (a gun-type warhead fired from the barrel of a
much larger gun). Such warheads were deployed by the U.S. until 1992,
accounting for a significant fraction of the U-235 in the arsenal.

Implosion type weapon

Fat Man, the Nagasaki bomb, used 13.6 lb (6.2 kg) of Pu-239, which is only
39% of bare-metal critical mass. (See Fat Man article for a detailed drawing.) The U-238 reflected,
13.6 lb pit was sub-critical before detonation. During detonation, criticality
was achieved by implosion. The plutonium pit was squeezed to increase its
density by simultaneous detonation of conventional explosives placed uniformly
around the pit. The explosives were detonated by multiple exploding-bridgewire detonators. It is
estimated that only about 20% of the plutonium underwent fission, the rest
(about 11 lbs) was scattered.

An implosion shock wave might be of such short duration that only a fraction
of the pit is compressed at any instant as the wave passes through it. A pusher
shell made out of low densitymetal—such as aluminum, beryllium, or
an alloy of
the two metals (aluminum being easier and safer to shape and beryllium for its
high-neutron-reflective capability) —may be needed. The pusher is located
between the explosive lens and the tamper. It works by reflecting some of the
shock wave backwards, thereby having the effect of lengthening its duration.
Fat Man used an aluminum pusher.

The key to Fat Man's greater efficiency was the inward momentum of the
massive U-238 tamper (which did not undergo fission). Once the chain reaction
started in the plutonium, the momentum of the implosion had to be reversed
before expansion could stop the fission. By holding everything together for a
few hundred nanoseconds more, the efficiency was increased.

Plutonium pit

The core of an implosion weapon – the fissile material and any reflector or
tamper bonded to it – is known as the pit. Some weapons tested during the 1950s
used pits made with U-235 alone, or in composite with plutonium,[8] but
all-plutonium pits are the smallest in diameter and have been the standard
since the early 1960s.

Casting and then machining plutonium is difficult not only because of its
toxicity, but also because plutonium has many different metallic phases, also known as allotropes. As plutonium cools, changes in phase result in
distortion. This distortion is normally overcome by alloying it with 3–3.5
molar% (0.9–1.0% by weight) gallium which causes it to take up its delta phase over a
wide temperature range.[9] When
cooling from molten it then suffers only a single phase change, from epsilon to
delta, instead of the four changes it would otherwise pass through. Other
trivalentmetals would also
work, but gallium has a small neutron absorption cross section and helps protect the
plutonium against corrosion. A drawback is that gallium compounds themselves are
corrosive and so if the plutonium is recovered from dismantled weapons for
conversion to plutonium dioxide for
power reactors, there is the difficulty of removing
the gallium.

Because plutonium is chemically reactive and toxic if inhaled or enters the
body by any other means, for protection of the assembler, it is common to plate
the completed pit with a thin layer of inert metal. In the first weapons,
nickel was
used but gold
is now preferred.[10]

Levitated-pit implosion

The first improvement on the Fat Man design was to put an air space between
the tamper and the pit to create a hammer-on-nail impact. The pit, sitting on a
hollow cone inside the tamper cavity, was said to be levitated. The three tests
of Operation Sandstone, in 1948, used Fat Man designs
with levitated pits. The largest yield was 49 kilotons, more than twice the
yield of the unlevitated Fat Man.[11]

It was immediately clear that implosion was the best design for a fission
weapon. Its only drawback seemed to be its diameter. Fat Man was 5 feet wide vs
2 feet for Little Boy.

Eleven years later, implosion designs had advanced sufficiently that the 5
foot-diameter sphere of Fat Man had been reduced to a 1 foot-diameter cylinder
2 feet long, the Swan device.

The Pu-239 pit of Fat Man was only 3.6 inches in diameter, the size of a
softball. The bulk of Fat Man's girth was the implosion mechanism, namely
concentric layers of U-238, aluminum, and high explosives. The key to reducing
that girth was the two-point implosion design.

Two-point linear implosion

A very inefficient implosion design is one that simply reshapes an ovoid
into a sphere, with minimal compression. In linear implosion, an untamped,
solid, elongated mass of Pu-239, larger than critical mass in a sphere, is
imbedded inside a cylinder of high explosive with a detonator at each
end.[12]

Detonation makes the pit critical by driving the ends inward, creating a
spherical shape. The shock may also change plutonium from delta to alpha phase,
increasing its density by 23%, but without the inward momentum of a true
implosion. The lack of compression makes it inefficient, but the simplicity and
small diameter make it suitable for use in artillery shells and atomic
demolition munitions - ADMs - also known as backpack or suitcase nukes.

All such low-yield battlefield weapons, whether gun-type U-235 designs or
linear implosion Pu-239 designs, pay a high price in fissile material in order
to achieve diameters between six and ten inches.

Two-point hollow-pit implosion

A more efficient two-point implosion system uses two high explosive lenses
and a hollow pit.

A hollow plutonium pit was the original plan for the 1945 Fat Man bomb, but
there was not enough time to develop and test the implosion system for it. A
simpler solid-pit design was considered more reliable, given the time
restraint, but it required a heavy U-238 tamper, a thick aluminum pusher, and
three tons of high explosives.

After the war, interest in the hollow pit design was revived. Its obvious
advantage is that a hollow shell of plutonium, shock-deformed and driven inward
toward its empty center, would carry momentum into its violent assembly as a
solid sphere. It would be self-tamping, requiring a smaller U-238 tamper, no
aluminum pusher, and less high explosive. The hollow pit made levitation
obsolete.

The Fat Man bomb had two concentric, spherical shells of high explosives,
each about 10 inches thick. The inner shell drove the implosion. The outer
shell consisted of a soccer-ball pattern of 32 high explosive lenses,
each of which converted the convex wave from its detonator into a concave wave
matching the contour of the outer surface of the inner shell. If these 32
lenses could be replaced with only two, the high explosive sphere could become
an ellipsoid (prolate spheroid) with a much smaller diameter.

The best illustration of these two features is a 1956 drawing from the
Swedish nuclear bomb program. The program was terminated before it produced a
test explosion. The drawing shows the essential elements of the two-point
hollow-pit design.

There are similar drawings in the open literature that come from the
post-war German nuclear bomb program, which was also terminated, and from the
French program, which produced an arsenal.

The mechanism of the high explosive lens (diagram item #6) is not shown in
the Swedish drawing, but a standard lens made of fast and slow high explosives,
as in Fat Man, would be much longer than the shape depicted. For a single high
explosive lens to generate a concave wave that envelops an entire hemisphere,
it must either be very long or the part of the wave on a direct line from the
detonator to the pit must be slowed dramatically.

A slow high explosive is too fast, but the flying plate of an "air lens" is
not. A metal plate, shock-deformed, and pushed across an empty space can be
designed to move slowly enough.[13][14] A two-point implosion
system using air lens technology can have a length no more than twice its
diameter, as in the Swedish diagram above.

Fusion-boosted fission weapons

The next step in miniaturization was to speed up the fissioning of the pit
to reduce the amount of time inertial confinement needed. The hollow pit
provided an ideal location to introduce fusion for the boosting of fission. A
50-50 mixture of tritium and deuterium gas, pumped into the pit during arming,
will fuse into helium and release free neutrons soon after fission begins. The
neutrons will start a large number of new chain reactions while the pit is
still critical.

Once the hollow pit is perfected, there is little reason not to boost.

The concept of fusion-boosted fission was first tested on May 25, 1951, in
the Item shot of Operation Greenhouse, Eniwetok, yield 45.5
kilotons.

Boosting reduces diameter in three ways, all the result of faster
fission:

Since the compressed pit does not need to be held together as long, the
massive U-238 tamper can be replaced by a light-weight beryllium shell (to
reflect escaping neutrons back into the pit). The diameter is reduced.

The mass of the pit can be reduced by half, without reducing yield.
Diameter is reduced again.

Since the mass of the metal being imploded (tamper plus pit) is reduced, a
smaller charge of high explosive is needed, reducing diameter even
further.

Since boosting is required to attain full design yield, any reduction in
boosting reduces yield. Boosted weapons are thus variable-yield weapons. Yield can be reduced any time
before detonation, simply by putting less than the full amount of tritium into
the pit during the arming procedure.

The first device whose dimensions suggest employment of all these features
(two-point, hollow-pit, fusion-boosted implosion) was the Swan device, tested
June 22, 1956, as the Inca shot of Operation Redwing, at Eniwetok. Its yield was 15
kilotons, about the same as Little Boy, the Hiroshima bomb. It weighed 105 lb
(47.6 kg) and was cylindrical in shape, 11.6 inches (29.5 cm) in diameter and
22.9 inches (58 cm) long. The above schematic illustrates what were probably
its essential features.

Eleven days later, July 3, 1956, the Swan was test-fired again at Eniwetok,
as the Mohawk shot of Redwing. This time it served as the primary, or first
stage, of a two-stage thermonuclear device, a role it played in a dozen such
tests during the 1950s. Swan was the first off-the-shelf, multi-use primary,
and the prototype for all that followed.

After the success of Swan, 11 or 12 inches seemed to become the standard
diameter of boosted single-stage devices tested during the 1950s. Length was
usually twice the diameter, but one such device, which became the W54 warhead, was closer
to a sphere, only 15 inches long. It was tested two dozen times in the 1957-62
period before being deployed. No other design had such a long string of test
failures. Since the longer devices tended to work correctly on the first try,
there must have been some difficulty in flattening the two high explosive
lenses enough to achieve the desired length-to-width ratio.

Another benefit of boosting, in addition to making weapons smaller, lighter,
and with less fissile material for a given yield, is that it renders weapons
immune to radiation interference (RI). It was discovered in the mid-1950s that
plutonium pits would be particularly susceptible to partial pre-detonation if
exposed to the intense radiation of a nearby nuclear explosion (electronics
might also be damaged, but this was a separate issue). RI was a particular
problem before effective early warning radar systems because a first strike
attack might make retaliatory weapons useless. Boosting reduces the amount of
plutonium needed in a weapon to below the quantity which would be vulnerable to
this effect.

Two-stage thermonuclear weapons

Pure fission or fusion-boosted fission weapons can be made to yield hundreds
of kilotons, at great expense in fissile material and tritium, but by far the
most efficient way to increase nuclear weapon yield beyond ten or so kilotons
is to tack on a second independent stage, called a secondary.

In the 1940s, bomb designers at Los Alamos thought the secondary would be a
canister of deuterium in liquified or hydride form. The fusion reaction would
be D-D, harder to achieve than D-T, but more affordable. A fission bomb at one
end would shock-compress and heat the near end, and fusion would propagate
through the canister to the far end. Mathematical simulations showed it
wouldn't work, even with large amounts of prohibitively expensive tritium added
in.

The entire fusion fuel canister would need to be enveloped by fission
energy, to both compress and heat it, as with the booster charge in a boosted
primary. The design breakthrough came in January of 1951, when Edward Teller and Stanisław Ulam invented radiation implosion - for
nearly three decades known publicly only as the Teller-Ulam H-bomb secret.

The concept of radiation implosion was first tested on May 9, 1951, in the
George shot of Operation Greenhouse, Eniwetok, yield 225 kilotons.
The first full test was on November 1, 1952, the Mike shot
of Operation Ivy, Eniwetok, yield 10.4 megatons.

In radiation implosion, the burst of x-ray energy coming from an exploding
primary is captured and contained within an opaque-walled radiation channel
which surrounds the nuclear energy components of the secondary. For a millionth
of a second, most of the energy of several kilotons of TNT is absorbed by a
plasma (superheated gas) generated from plastic foam in the radiation channel.
With energy going in and not coming out, the plasma rises to solar core
temperatures and expands with solar core pressures. Nearby objects which are
still cool are crushed by the temperature difference.

The cool nuclear materials surrounded by the radiation channel are imploded
much like the pit of the primary, except with vastly more force. This greater
pressure enables the secondary to be significantly more powerful than the
primary, without being much larger.

For example, for the Redwing Mohawk test on July 3, 1956, a secondary called
the Flute was attached to the Swan primary. The Flute was 15 inches (38 cm) in
diameter and 23.4 inches (59 cm) long, about the size of the Swan. But it
weighed ten times as much and yielded 24 times as much energy (355 kilotons, vs
15 kilotons).

Equally important, the active ingredients in the Flute probably cost no more
than those in the Swan. Most of the fission came from cheap U-238, and the
tritium was manufactured in place during the explosion. Only the spark plug at
the axis of the secondary needed to be fissile.

A spherical secondary can achieve higher implosion densities than a
cylindrical secondary, because spherical implosion pushes in from all
directions toward the same spot. However, in warheads yielding more than one
megaton, the diameter of a spherical secondary would be too large for most
applications. A cylindrical secondary is necessary in such cases. The small,
cone-shaped re-entry vehicles in multiple-warhead ballistic missiles after 1970
tended to have warheads with spherical secondaries, and yields of a few hundred
kilotons.

As with boosting, the advantages of the two-stage thermonuclear design are
so great that there is little incentive not to use it, once a nation has
mastered the technology.

In engineering terms, radiation implosion allows for the exploitation of
several known features of nuclear bomb materials which heretofore had eluded
practical application. For example:

The best way to store deuterium in a reasonably dense state is to
chemically bond it with lithium, as lithium deuteride. But the lithium-6
isotope is also the raw material for tritium production, and an exploding bomb
is a nuclear reactor. Radiation implosion will hold everything together long
enough to permit the complete conversion of lithium-6 into tritium, while the
bomb explodes. So the bonding agent for deuterium permits use of the D-T fusion
reaction without any pre-manufactured tritium being stored in the secondary.
The tritium production constraint disappears.

For the secondary to be imploded by the hot, radiation-induced plasma
surrounding it, it must remain cool for the first microsecond, i.e., it must be
encased in a massive radiation (heat) shield. The shield's massiveness allows
it to double as a tamper, adding momentum and duration to the implosion. No
material is better suited for both of these jobs than ordinary, cheap
uranium-238, which happens, also, to undergo fission when struck by the
neutrons produced by D-T fusion. This casing, called the pusher, thus has three
jobs: to keep the secondary cool, to hold it, inertially, in a highly
compressed state, and, finally, to serve as the chief energy source for the
entire bomb. The consumable pusher makes the bomb more a uranium fission bomb
than a hydrogen fusion bomb. It is noteworthy that insiders never used the term
hydrogen bomb.

Finally, the heat for fusion ignition comes not from the primary but from a
second fission bomb called the spark plug, imbedded in the heart of the
secondary. The implosion of the secondary implodes this spark plug, detonating
it and igniting fusion in the material around it, but the spark plug then
continues to fission in the neutron-rich environment until it is fully
consumed, adding significantly to the yield.[15]

The initial impetus behind the two-stage weapon was President Truman's 1950
promise to build a 10-megaton hydrogen superbomb as America's response to the
1949 test of the first Soviet fission bomb. But the resulting invention turned
out to be the cheapest and most compact way to build small nuclear bombs as
well as large ones, erasing any meaningful distinction between A-bombs and
H-bombs, and between boosters and supers. All the best techniques for fission
and fusion explosions are incorporated into one all-encompassing,
fully-scalable design principle. Even six-inch diameter nuclear artillery
shells can be two-stage thermonuclears.

In the ensuing fifty years, nobody has come up with a better way to build a
nuclear bomb. It is the design of choice for the U.S., Russia, Britain, France,
and China, the five thermonuclear powers. The other nuclear-armed nations,
Israel, India, Pakistan, and North Korea, probably have single-stage weapons,
possibly boosted.

Interstage

In a two-stage thermonuclear weapon, three types of energy emerge from the
primary to impact the secondary: the expanding hot gases from high explosive
charges which implode the primary, plus the electromagnetic radiation and the
neutrons from the primary's nuclear detonation. An essential energy transfer
modulator called the interstage, between the primary and the secondary,
protects the secondary from the hot gases and channels the electromagnetic
radiation and neutrons toward the right place at the right time.

There is very little information in the open literature about the mechanism
of the interstage. Its first mention in a U.S. government document formally
released to the public appears to be a caption in a recent graphic promoting
the Reliable Replacement Warhead Program. If built, this new design would
replace "toxic, brittle material" and "expensive 'special' material" in the
interstage.[16] This statement suggests
the interstage may contain beryllium to moderate the flux of neutrons from the
primary, and perhaps something to absorb and re-radiate the x-rays in a
particular manner.[17]

The interstage and the secondary are encased together inside a stainless
steel membrane to form the canned subassembly (CSA), an arrangement which has
never been depicted in any open-source drawing.[18] The most detailed illustration of an interstage shows a British
thermonuclear weapon with a cluster of items between its primary and a
cylindrical secondary. They are labeled "end-cap and neutron focus lens,"
"reflector/neutron gun carriage," and "reflector wrap." The origin of the
drawing, posted on the internet by Greenpeace, is uncertain, and there is no
accompanying explanation.[19]

Specific designs

While every nuclear weapon design falls into one of the above categories,
specific designs have occasionally become the subject of news accounts and
public discussion, often with incorrect descriptions about how they work and
what they do. Examples:

Hydrogen bombs

All modern nuclear weapons make some use of D-T fusion. Even pure fission
weapons include neutron generators which are high-voltage vacuum tubes
containing trace amounts of tritium and deuterium.

However, in the public perception, hydrogen bombs, or H-bombs, are
multi-megaton devices a thousand times more powerful than Hiroshima's Little
Boy. Such high-yield bombs are actually two-stage thermonuclears, scaled up to
the desired yield, with uranium fission, as usual, providing most of their
destructive energy.

The idea of the hydrogen bomb first came to public attention in 1949, when
prominent scientists openly recommended against building nuclear bombs more
powerful than the standard pure-fission model, on both moral and practical
grounds. Their assumption was that critical mass considerations would limit the
potential size of fission explosions, but that a fusion explosion could be as
large as its supply of fuel, which has no critical mass limit. In 1949, the
Russians exploded their first fission bomb, and in 1950 President Truman ended
the H-bomb debate by ordering the Los Alamos designers to build one.

In 1952, the 10.4-megaton Ivy Mike explosion was announced as the first
hydrogen bomb test, reinforcing the idea that hydrogen bombs are a thousand
times more powerful than fission bombs.

In 1954, J. Robert Oppenheimer was labeled a hydrogen
bomb opponent. The public did not know there were two kinds of hydrogen bomb
(neither of which is accurately described as a hydrogen bomb). On May 23, when
his security clearance was revoked, item three of the four public findings
against him was "his conduct in the hydrogen bomb program." In 1949,
Oppenheimer had supported single-stage fusion-boosted fission bombs, to
maximize the explosive power of the arsenal given the trade-off between
plutonium and tritium production. He opposed two-stage thermonuclear bombs
until 1951, when radiation implosion, which he called "technically sweet,"
first made them practical. He no longer objected. The complexity of his
position was not revealed to the public until 1976, thirteen years after his
death.[20]

When ballistic missiles replaced bombers in the 1960s, most multi-megaton
bombs were replaced by missile warheads (also two-stage thermonuclears) scaled
down to one megaton or less.

Alarm Clock/Sloika

The first effort to exploit the symbiotic relationship between fission and
fusion was a 1940s design that mixed fission and fusion fuel in alternating
thin layers. As a single-stage device, it would have been a cumbersome
application of boosted fission. It first became practical when incorporated
into the secondary of a two-stage thermonuclear weapon.[21]

The U.S. name, Alarm Clock, was a nonsense code name. The Russian name for
the same design was more descriptive: Sloika, a layered pastry cake. A
single-stage Russian Sloika was tested on August 12, 1953. No single-stage U.S.
version was tested, but the Union shot of Operation Castle, April 26, 1954, was
a two-stage thermonuclear code-named Alarm Clock. Its yield, at Bikini, was 6.9
megatons.

Because the Russian Sloika test used dry lithium-6 deuteride eight months
before the first U.S. test to use it (Castle Bravo, March 1, 1954), it was
sometimes claimed that Russia won the H-bomb race. (The 1952 U.S. Ivy Mike test
used cryogenically-cooled liquid deuterium as the fusion fuel in the secondary,
and employed the D-D fusion reaction.) However, the first Russian test to use a
radiation-imploded secondary, the essential feature of a true H-bomb, was on
November 23, 1955, three years after Ivy Mike.

Clean bombs

Bassoon, the prototype for a 3.5-megaton clean
bomb or a 25-megaton dirty bomb. Dirty version shown here, before its 1956
test.

On March 1, 1954, America's largest-ever nuclear test explosion, the
15-megaton Bravo shot of Operation Castle at Bikini, delivered a
promptly lethal dose of fission-product fallout to more than 6,000 square miles
of Pacific Ocean surface.[22] Radiation
injuries to Marshall Islanders and Japanese fishermen made that fact public and
revealed the role of fission in hydrogen bombs.

In response to the public alarm over fallout, an effort was made to design a
clean multi-megaton weapon, relying almost entirely on fusion. Since it takes
roughly five megatons of fusion to produce the same blast and fire effect as
one megaton of fission, the clean bomb needed to be very large. For the first
and only time, a third stage, called the tertiary, was added, using the
secondary as its primary. The device was called Bassoon. It was tested as the
Zuni shot of Operation Redwing, at Bikini on May 28, 1956. With all the uranium
in Bassoon replaced with a substitute material such as lead, its yield was 3.5
megatons, 85% fusion and only 15% fission.

On July 19, AEC Chairman Lewis Strauss said the clean bomb test "produced
much of importance . . . from a humanitarian aspect." However, two days later
the dirty version of Bassoon, with the uranium parts restored, was tested as
the Tewa shot of Redwing. Its 5-megaton yield, 87% fission, was deliberately
suppressed to keep fallout within a smaller area. This dirty version was later
deployed as the three-stage, 25-megaton Mark-41 bomb, which was carried by U.S.
Air Force bombers, but never tested at full yield.

As such, high-yield clean bombs were a public relations exercise. The actual
deployed weapons were the dirty version, which maximized yield for the same
size device.

Cobalt bombs

A fictional doomsday bomb, made popular by Neville Shute's 1957 novel, and subsequent 1959 movie, On the Beach, the cobalt bomb was a
hydrogen bomb with a jacket of cobalt metal. The neutron-activated cobalt would
supposedly have maximized the environmental damage from radioactive fallout.
This bomb was popularized as the 'Doomsday Device' in the 1964 film 'Dr.
Strangelove or: How I Learned to Stop Worrying and Love the Bomb' in the film
the bomb brings about the end of mankind by covering the planet in a
radioactive shroud for 93 years. The element added to the bombs is referred to
in the film as 'cobalt-chlorium G'

Such "salted" weapons were requested by the U.S. Air Force and seriously
investigated, possibly built and tested, but not deployed. In the 1964 edition
of the DOD/AEC book The Effects of Nuclear Weapons, a new section titled
Radiological Warfare clarified the issue.[23] Fission products are as deadly as neutron-activated cobalt. The
standard high-fission thermonuclear weapon is automatically a weapon of
radiological warfare, as dirty as a cobalt bomb.

Initially, gamma radiation from the fission products from an equivalent size
fission-fusion-fission bomb are much more intense than Co-60: 15,000 times more
intense at 1 hour; 35 times more intense at 1 week; 5 times more intense at 1
month; and about equal at 6 months. Thereafter fission drops off rapidly so
that Co-60 fallout is 8 times more intense than fission at 1 year and 150 times
more intense at 5 years. The very long lived isotopes produced by fission would
overtake the 60Co again after about 75 years. [24]

Fission-fusion-fission bombs

In 1954, to explain the surprising amount of fission-product fallout
produced by hydrogen bombs, Ralph Lapp coined the term fission-fusion-fission
to describe a process inside what he called a three-stage thermonuclear weapon.
His process explanation was correct, but his choice of terms caused confusion
in the open literature. The stages of a nuclear weapon are not fission, fusion,
and fission. They are the primary, the secondary, and, in one exceptionally
powerful weapon, the tertiary. Each of these stages employs fission, fusion,
and fission.

Neutron bombs

While high-yield clean bombs were never deployed, some low-yield clean bombs
were. Officially known as enhanced radiation weapons, ERWs, they are more
accurately described as suppressed yield weapons. When the yield of a nuclear
weapon is less than one kiloton, its lethal radius from blast, 700 m (2300 ft),
is less than that from its neutron radiation. If a one-kiloton ERW is exploded
800 m above ground, buildings at ground zero will survive but people in them
will die of radiation illness caused by neutrons and other fireball
radiation.

Although the buildings would survive the blast, neutron activation would
make them radioactive. If detonation occurred at a lower altitude, the full
force of one kiloton (i.e., four thousand 500 lb bombs) would flatten them.

ERWs were two-stage thermonuclears with all non-essential uranium removed to
minimize fission yield. Fusion provided the neutrons. Developed in the 1950s,
they were first deployed in the 1970s, by U.S. forces in Europe. The last ones
were retired in the 1990s.

Samuel Cohen in 1958 investigated a low-yield 'clean'
nuclear weapon and discovered that the 'clean' bomb case thickness scales as
the cube-root of yield. So a larger percentage of neutrons escapes from a small
detonation, due to the thinner case required to reflect back X-rays during the
secondary stage ignition. For example, a 1-kiloton bomb only needs a case
1/10th the thickness of that required for 1-megaton.

So although most neutrons are absorbed by the casing in a 1-megaton bomb, in
a 1-kiloton bomb they would mostly escape. A neutron bomb is only feasible if
the yield is sufficiently high that efficient fusion stage ignition is
possible, and if the yield is low enough that the case thickness will not
absorb too many neutrons. This means that neutron bombs have a yield range of
1-10 kilotons, with fission proportion varying from 50% at 1-kiloton to 25% at
10-kilotons (all of which comes from the primary stage). The neutron output per
kiloton is then 10-15 times greater than for a pure fission implosion weapon or
for a strategic warhead like a W87 or W88. [25]

Oralloy thermonuclear warheads

In 1999, nuclear weapon design was in the news again, for the first time in
decades. In January, the U.S. House of Representatives released the Cox
Report (Christopher Cox R-CA) which alleged that
China had somehow acquired classified information about the U.S. W88 warhead. Nine months
later, Wen Ho Lee, a Taiwanese immigrant working at Los Alamos, was publicly accused of
spying, arrested, and served nine months in pre-trial detention, before the case against him
was dismissed. It is not clear that there was, in fact, any espionage.

In the course of eighteen months of news coverage, the W88 warhead was
described in unusual detail. The New York Times printed a schematic diagram on its
front page.[26] The most detailed drawing
appeared in A Convenient Spy, the 2001 book on the Wen Ho Lee case by Dan
Stober and Ian Hoffman, adapted and shown here with permission.

Designed for use on Trident II (D-5) submarine-launched ballistic
missiles, the W88 entered service in 1990 and was the last warhead designed
for the U.S. arsenal. It has been described as the most advanced, although open
literature accounts do not indicate any major design features that were not
available to U.S. designers in 1958.

The above diagram shows all the standard features of ballistic missile
warheads since the 1960s, with two exceptions that give it a higher yield for
its size.

The outer layer of the secondary, called the "pusher," which serves three
functions: heat shield, tamper, and fission
fuel, is made of U-235 instead of U-238, hence the name Oralloy (U-235) Thermonuclear. Being fissile, rather than merely
fissionable, allows the pusher to fission faster and more completely,
increasing yield. This feature is available only to nations with a great wealth
of fissile uranium. The U.S. is estimated to have 500 tons.

The secondary is located in the wide end of the re-entry cone, where it can
be larger, and thus more powerful. The usual arrangement is to put the heavier,
denser secondary in the narrow end for greater aerodynamic stability during
re-entry from outer space, and to allow more room for a bulky primary in the
wider part of the cone. (The W87 warhead drawing in the previous section shows
the usual arrangement.) Because of this new geometry, the W88 primary uses
compact conventional high explosives (CHE) to save space,[27] rather than the more usual, and bulky but
safer, insensitive high explosives (IHE). The re-entry cone probably has
ballast in the nose for aerodynamic stability.[28]

Notice that the alternating layers of fission and fusion material in the
secondary are an application of the Alarm Clock/Sloika principle.

Reliable replacement warhead

The United States has not produced any nuclear warheads since 1989, when the
Rocky Flats pit production plant, near Boulder, Colorado, was shut down for environmental
reasons. With the end of the Cold War coming two years later, the production
line has remained idle except for inspection and maintenance functions.

The National Nuclear Security
Administration, the latest successor for nuclear weapons to the Atomic Energy Commission and
the Department of Energy, has
proposed building a new pit facility and starting the production line for a new
warhead called the Reliable Replacement Warhead (RRW).[29] Two advertised safety improvements of the RRW
would be a return to the use of "insensitive high explosives which are far less
susceptible to accidental detonation," and the elimination of "certain
hazardous materials, such as beryllium, that are harmful to people and the
environment."[30] Since the new warhead
would not require any nuclear testing, it could not use a new design with
untested concepts.

The Weapon Design Laboratories

Berkeley

The first systematic exploration of nuclear weapon design concepts took
place in the summer of 1942 at the University of California,
Berkeley. Important early discoveries had been made at the adjacent
Lawrence Berkeley
Laboratory, such as the 1940 production and isolation of plutonium. A
Berkeley professor, J. Robert Oppenheimer, had just
been hired to run the nation's secret bomb design effort. His first act was to
convene the 1942 summer conference.

By the time he moved his operation to the new secret town of Los Alamos, New
Mexico, in the spring of 1943, the accumulated wisdom on nuclear weapon design
consisted of five lectures by Berkeley professor Robert Serber, transcribed and distributed as the Los Alamos Primer. The Primer addressed fission energy,
neutron
production and capture, nuclear chain reactions, critical
mass, tampers, predetonation, and three methods of assembling a bomb: gun
assembly, implosion, and "autocatalytic methods," the one approach that turned
out to be a dead end.

Los Alamos

At Los Alamos, it was found in April 1944 by Emilio G. Segrè that the proposed Thin Man Gun assembly type bomb would not work for
plutonium because of predetonation problems caused by Pu-240 impurities. So
Fat
Man the Implosion type bomb was given high priority as the only option for
plutonium. The Berkeley discussions had generated theoretical estimates of
critical mass, but nothing precise. The main wartime job at Los Alamos was the
experimental determination of critical mass, which had to wait until sufficient
amounts of fissile material arrived from the production plants: uranium from
Oak Ridge, Tennessee, and plutonium from the
Hanford site in Washington.

In 1945, using the results of critical mass experiments, Los Alamos
technicians fabricated and assembled components for four bombs: the TrinityGadget, Little
Boy, Fat Man, and an unused spare Fat Man. After the war, those who
could, including Oppenheimer, returned to university teaching positions. Those
who remained worked on levitated and hollow pits and conducted weapon effects
tests such as Crossroads Able and Baker at Bikini
Atoll in 1946.

All of the essential ideas for incorporating fusion into nuclear weapons
originated at Los Alamos between 1946 and 1952. After the Teller-Ulam radiation implosion breakthrough of 1951,
the technical implications and possibilities were fully explored, but ideas not
directly relevant to making the largest possible bombs for long-range Air Force
bombers were shelved.

Because of Oppenheimer's initial position in the H-bomb debate, in
opposition to large thermonuclear weapons, and the assumption that he still had
influence over Los Alamos despite his departure, political allies of Edward Teller decided he needed his own laboratory in order
to pursue H-bombs. By the time it was opened in 1952, in Livermore, California, Los
Alamos had finished the job Livermore was designed to do.

Livermore

With its original mission no longer available, the Livermore lab tried
radical new designs, that failed. Its first three nuclear tests were fizzles:
in 1953, two single-stage fission devices with uranium hydride pits, and in
1954, a two-stage thermonuclear device in which the secondary heated up
prematurely, too fast for radiation implosion to work properly.

Shifting gears, Livermore settled for taking ideas Los Alamos had shelved
and developing them for the Army and Navy. This led Livermore to specialize in
small-diameter tactical weapons, particularly ones using two-point implosion
systems, such as the Swan. Small-diameter tactical weapons became primaries for
small-diameter secondaries. Around 1960, when the superpower arms race became a
ballistic missile race, Livermore warheads were more useful than the large,
heavy Los Alamos warheads. Los Alamos warheads were used on the first intermediate-range ballistic
missiles, IRBMs, but smaller Livermore warheads were used on the first
intercontinental ballistic
missiles, ICBMs, and submarine-launched ballistic
missiles, SLBMs, as well as on the first multiple warhead
systems on such missiles.[31]

In 1957 and 1958 both labs built and tested as many designs as possible, in
anticipation that a planned 1958 test ban might become permanent. By the time
testing resumed in 1961 the two labs had become duplicates of each other, and
design jobs were assigned more on workload considerations than lab specialty.
Some designs were horse-traded. For example, the W38 warhead for the
Titan I missile started out as a Livermore project,
was given to Los Alamos when it became the Atlas missile warhead, and in 1959 was given back to
Livermore, in trade for the W54Davy Crockett warhead, which went
from Livermore to Los Alamos.

The period of real innovation was ending by then, anyway. Warhead designs
after 1960 took on the character of model changes, with every new missile
getting a new warhead for marketing reasons. The chief substantive change
involved packing more fissile uranium into the secondary, as it became
available with continued uranium enrichment and the dismantlement of the large
high-yield bombs.

Explosive testing

Nuclear weapons are designed by trial and error. The trial often involves
exploding a prototype.

In a nuclear explosion, a large number of discrete events, with various
probabilities, aggregate into short-lived, chaotic energy flows inside the
device casing. Complex mathematical models are required to approximate the
processes, and in the 1950s there were no computers powerful enough to run them
properly. Even today's computers and their codes are not fully
adequate.[32]

It was easy enough to design reliable weapons for the stockpile. If the
prototype worked, it could be weaponized and mass produced.

It was much more difficult to understand how it worked or why it failed.
Designers gathered as much data as possible during the explosion, before the
device destroyed itself, and used the data to calibrate their models, often by
inserting fudge factors into equations to make the simulations match
experimental results. They also analyzed the weapon debris in fallout to see
how much of a potential nuclear reaction had taken place.

Light pipes

An important tool for test analysis was the diagnostic light pipe. A probe
inside a test device could transmit information by heating a plate of metal to
incandescence, an event that could be recorded at the far end of a long, very
straight pipe.

The picture below shows the Shrimp device, detonated on March 1, 1954 at
Bikini, as the Castle Bravo test. Its 15-megaton explosion was the
largest ever by the United States. The silhouette of a man is shown for scale.
The device is supported from below, at the ends. The pipes going into the shot
cab ceiling, which appear to be supports, are diagnostic light pipes. The eight
pipes at the right end (1) sent information about the detonation of the
primary. Two in the middle (2) marked the time when x-radiation from the
primary reached the radiation channel around the secondary. The last two pipes
(3) noted the time radiation reached the far end of the radiation channel, the
difference between (2) and (3) being the radiation transit time for the
channel.[33]

From the shot cab, the pipes turned horizontal and traveled 7500 ft (2.3
km), along a causeway built on the Bikini reef, to a remote-controlled data
collection bunker on Namu Island.

While x-rays would normally travel at the speed of light through a low
density material like the plastic foam channel filler between (2) and (3), the
intensity of radiation from the exploding primary created a relatively opaque
radiation front in the channel filler which acted like a slow-moving logjam to
retard the passage of radiant energy. Behind this moving front was a
fully-ionized, low-z (low atomic number) plasma heated to 20,000 degrees
Celsius, soaking up energy like a black box, and eventually driving the
implosion of the secondary.[34]

The radiation transit time, on the order of half a microsecond, is the time
it takes the entire radiation channel to reach thermal equilibrium as the
radiation front moves down its length. The implosion of the secondary is based
on the temperature difference between the hot channel and the cool interior of
the secondary. Its timing is important because the interior of the secondary is
subject to neutron preheat.

While the radiation channel is heating and starting the implosion, neutrons
from the primary catch up with the x-rays, penetrate into the secondary and
start breeding tritium with the third reaction noted in the first section
above. This Li-6 + n reaction is exothermic, producing 5 Mev per event. The
spark plug is not yet compressed and thus is not critical, so there won't be
significant fission or fusion. But if enough neutrons arrive before implosion
of the secondary is complete, the crucial temperature difference will be
degraded. This is the reported cause of failure for Livermore's first
thermonuclear design, the Morgenstern device, tested as Castle
Koon, April 7, 1954.

These timing issues are measured by light-pipe data. The mathematical
simulations which they calibrate are called radiation flow hydrodynamics codes,
or channel codes. They are used to predict the effect of future design
modifications.

It is not clear from the public record how successful the Shrimp light pipes
were. The data bunker was far enough back to remain outside the mile-wide
crater, but the 15-megaton blast, two and a half times greater than expected,
breached the bunker by blowing its 20-ton door off the hinges and across the
inside of the bunker. (The nearest people were twenty miles farther away, in a
bunker that survived intact.)[35]

Fallout analysis

The most interesting data from Castle Bravo came from radio-chemical
analysis of weapon debris in fallout. Because of a shortage of enriched
lithium-6, 60% of the lithium in the Shrimp secondary was ordinary lithium-7,
which doesn't breed tritium as easily as lithium-6 does. But it does breed
lithium-6 as the product of an "n, 2n" reaction (one neutron in, two neutrons
out), a known fact, but with unknown probability. The probability turned out to
be high.

Fallout analysis revealed to designers that, with the n, 2n reaction, the
Shrimp secondary effectively had two and half times as much lithium-6 as
expected. The tritium, the fusion yield, the neutrons, and the fission yield
were all increased accordingly.[36]

As noted above, Bravo's fallout analysis also told the outside world, for
the first time, that thermonuclear bombs are more fission devices than fusion
devices. A Japanese fishing boat named the Lucky Dragon sailed home with enough fallout on
its decks to allow scientists in Japan and elsewhere to determine, and
announce, that most of the fallout had come from the fission of U-238 by
fusion-produced 14 MeV neutrons.

Underground testing

Subsidence Craters at Yucca Flat, Nevada
Test Site.

The global alarm over radioactive fallout, which began with the Castle Bravo
event, eventually drove nuclear testing underground. The last U.S. above-ground
test took place at Johnston Island on November 4, 1962. During the next three
decades, until September 23, 1992, the U.S. conducted an average of 2.4
underground nuclear explosions per month, all but a few at the Nevada Test Site (NTS) northwest of Las Vegas.

The Yucca Flat section of the NTS is covered with subsidence craters
resulting from the collapse of terrain over intensely radioactive underground
caverns created by nuclear explosions (see photo).

After the 1974 Threshold Test Ban Treaty (TTBT), which limited
underground explosions to 150 kilotons or less, warheads like the half-megaton
W88 had to be tested at less than full yield. Since the primary must be
detonated at full yield in order to generate data about the implosion of the
secondary, the reduction in yield had to come from the secondary. Replacing
much of the lithium-6 deuteride fusion fuel with lithium-7 hydride limited the
deuterium available for fusion, and thus the overall yield, without changing
the dynamics of the implosion. The functioning of the device could be evaluated
using light pipes, other sensing devices, and analysis of trapped weapon
debris. The full yield of the stockpiled weapon could be calculated by
extrapolation.

Production facilities

When two-stage weapons became standard in the early 1950s, weapon design
determined the layout of America's new, widely dispersed production facilities,
and vice versa.

Because primaries tend to be bulky, especially in diameter, plutonium is the
fissile material of choice for pits, with beryllium reflectors. It has a
smaller critical mass than uranium. The Rocky Flats plant in Boulder, Colorado, was built in
1952 for pit production and consequently became the plutonium and beryllium
fabrication facility.

The Y-12 plant in Oak Ridge, Tennessee,
where mass spectrometers called Calutrons had enriched uranium for the Manhattan Project, was redesigned to make secondaries.
Fissile U-235 makes the best spark plugs because its critical mass is larger,
especially in the cylindrical shape of early thermonuclear secondaries. Early
experiments used the two fissile materials in combination, as composite Pu-Oy
pits and spark plugs, but for mass production, it was easier to let the
factories specialize: plutonium pits in primaries, uranium spark plugs and
pushers in secondaries.

Y-12 made lithium-6 deuteride fusion fuel and U-238 parts, the other two
ingredients of secondaries.

The Savannah River plant in Aiken, South Carolina, also built in 1952, operated nuclear reactors which converted U-238 into Pu-239 for
pits, and lithium-6 (produced at Y-12) into tritium for booster gas. Since its
reactors were moderated with heavy water, deuterium oxide, it also made
deuterium for booster gas and for Y-12 to use in making lithium-6
deuteride.

Warhead design safety

Gun-type weapons

It is inherently dangerous to have a weapon containing a quantity and shape
of fissile material which can form a critical mass through a relatively simple
accident. Because of this danger, the high explosives in Little Boy (four bags
of Cordite powder) were inserted into the bomb in flight, shortly after takeoff
on August 6, 1945. It was the first time a gun-type nuclear weapon had ever
been fully assembled.

Neither of these effects is likely with implosion weapons since there is
normally insufficient fissile material to form a critical mass without the
correct detonation of the lenses. However, the earliest implosion weapons had
pits so close to criticality that accidental detonation with some nuclear yield
was a concern.

On August 9, 1945, Fat Man was loaded onto its airplane fully assembled, but
later, when levitated pits made a space between the pit and the tamper, it was
feasible to utilize in-flight pit insertion. The bomber would take off with no
fissile material in the bomb. Some older implosion-type weapons, such as the US
Mark 4 and Mark 5, used this system.

In-flight pit insertion will not work with a hollow pit in contact with its
tamper.

Steel ball safety method

A diagram of the Green
Grass warhead's steel ball-bearing safety device, shown left, filled (safe)
and right, empty (live). The steel balls were emptied into a hopper underneath
the aircraft before flight, the steel balls could be re-inserted using a funnel
by rotating the bomb on its trolley and raising the hopper.

As shown in the diagram, one method used to decrease the likelihood of
accidental detonation used metal balls. The balls were emptied into the pit;
this would prevent detonation by increasing density of the hollowed pit. This
design was used in the Green Grass weapon, also known as the Interim Megaton
Weapon and was also used in Violet
Club and the Yellow Sun Mk.1 bombs.

Chain safety method

Alternatively, the pit can be "safed" by having its normally-hollow core
filled with an inert material such as a fine metal chain, possibly made of
cadmium to absorb neutrons. While the chain is in the center of the pit, the
pit can't be compressed into an appropriate shape to fission; when the weapon
is to be armed, the chain is removed. Similarly, although a serious fire could
detonate the explosives, destroying the pit and spreading plutonium to
contaminate the surroundings as has happened in several weapons accidents, it could not
however, cause a nuclear explosion.

Wire safety method

The US W47
warhead used in Polaris A1 and Polaris A2 had a safety device consisting of a
boron-coated-wire inserted into the hollow pit at manufacture. The warhead was
armed by withdrawing the wire onto a spool driven by an electric motor.
However, once withdrawn the wire could not be re-inserted.[37]

One-point safety

While the firing of one detonator out of many will not cause a hollow pit to
go critical, especially a low-mass hollow pit that requires boosting, the
introduction of two-point implosion systems made that possibility a real
concern.

In a two-point system, if one detonator fires, one entire hemisphere of the
pit will implode as designed. The high-explosive charge surrounding the other
hemisphere will explode progressively, from the equator toward the opposite
pole. Ideally, this will pinch the equator and squeeze the second hemisphere
away from the first, like toothpaste in a tube. By the time the explosion
envelops it, its implosion will be separated both in time and space from the
implosion of the first hemisphere. The resulting dumbbell shape, with each end
reaching maximum density at a different time, may not become critical.

Unfortunately, it is not possible to tell on the drawing board how this will
play out. Nor is it possible using a dummy pit of U-238 and high-speed x-ray
cameras, although such tests are helpful. For final determination, a test needs
to be made with real fissile material. Consequently, starting in 1957, a year
after Swan, both labs began one-point safety tests.

Out of 25 one-point safety tests conducted in 1957 and 1958, seven had zero
or slight nuclear yield (success), three had high yields of 300 kt to 500 kt
(severe failure), and the rest had unacceptable yields between those
extremes.

Of particular concern was Livermore's W47 warhead for the Polaris submarine
missile. The last test before the 1958 moratorium was a one-point test of the
W47 primary, which had an unacceptably high nuclear yield of 400 lb of TNT
equivalent (Hardtack II Titania). With the test moratorium in force, there was
no way to refine the design and make it inherently one-point safe. Los Alamos
had a suitable primary that was one-point safe, but rather than share with Los
Alamos the credit for designing the first SLBM warhead, Livermore chose to use
mechanical safing on its own inherently unsafe primary. The wire safety scheme
described above was the result.[38]

It turns out that the W47 may have been safer than anticipated. The wire-safety system may
have rendered most of the warheads "duds," unable to fire when detonated.

When testing resumed in 1961, and continued for three decades, there was
sufficient time to make all warhead designs inherently one-point safe, without
need for mechanical safing.

In addition to the above steps to reduce the probability of a nuclear
detonation arrising from a single fault, locking mechanisms referred to by NATO
states as Permissive Action Links are sometimes attached to the control
mechanisms for nuclear warheads. Permissive Action Links act solely to prevent
an unauthorised use of a nuclear weapon.

References

Specific

^ The physics package
is the nuclear explosive module inside the bomb casing, missile warhead, or
artillery shell, etc., which delivers the weapon to its target. While
photographs of weapon casings are common, photographs of the physics package
are quite rare, even for the oldest and crudest nuclear weapons. For a
photograph of a modern physics package see W80.

^ The United States and
the Soviet Union were the only nations to build large nuclear arsenals with
every possible type of nuclear weapon. The U.S. had a four-year head start and
was the first to produce fissile material and fission weapons, all in 1945. The
only Soviet claim for a design first was the Joe 4 detonation on
August 12, 1953, said to be the first deliverable hydrogen bomb. However, as
Herbert York first revealed in The Advisors: Oppenheimer, Teller and the
Superbomb (W.H. Freeman, 1976), it was not a true hydrogen bomb (it was a
boosted fission weapon of the Sloika/Alarm Clock type, not a two-stage
thermonuclear). Soviet dates for the essential elements of warhead
miniaturization – boosted, hollow-pit, two-point, air lens primaries – are not
available in the open literature, but the larger size of Soviet ballistic
missiles is often explained as evidence of an initial Soviet difficulty in
miniaturizing warheads.

^ The main source for
this section is Samuel Glasstone and Philip Dolan, The Effects of Nuclear
Weapons, Third Edition, 1977, U.S. Dept of Defense and U.S. Dept of Energy (see
links in General References, below), with the same information in more detail
in Samuel Glasstone, Sourcebook on Atomic Energy, Third Edition, 1979, U.S.
Atomic Energy Commission, Krieger Publishing.

^ Samuel Glasstone,
The Effects of Nuclear Weapons, 1962, Revised 1964, U.S. Dept of Defense and
U.S. Dept of Energy, pp.464-5. This section was removed from later editions,
but, according to Glasstone in 1978, not because it was inaccurate or because
the weapons had changed.

^ Broad, William J.
(7 September 1999), "Spies versus sweat, the debate over China's nuclear
advance," New York Times, p 1. The front page drawing was
similar to one that appeared four months earlier in the the San Jose Mercury
News.

^ Sybil Francis,
Warhead Politics: Livermore and the Competitive System of Nuclear Warhead
Design, UCRL-LR-124754, June 1995, Ph.D. Dissertation, Massachusetts Institute
of Technology, available from National Technical Information Service. This
233-page thesis was written by a weapons-lab outsider for public distribution.
The author had access to all the classified information at Livermore that was
relevant to her research on warhead design; consequently, she was required to
use non-descriptive code words for certain innovations.

^ Walter Goad,
Declaration for the
Wen Ho Lee case, May 17, 2000. Goad began thermonuclear weapon design work
at Los Alamos in 1950. In his Declaration, he mentions "basic scientific
problems of computability which cannot be solved by more computing power alone.
These are typified by the problem of long range predictions of weather and
climate, and extend to predictions of nuclear weapons behavior. This accounts
for the fact that, after the enormous investment of effort over many years,
weapons codes can still not be relied on for significantly new designs."

^ The public
literature mentions three different force mechanism for this implosion:
radiation pressure, plasma pressure, and explosive ablation of the outer
surface of the secondary pusher. All three forces are present; and the relative
contribution of each is one of the things the computer simulations try to
explain. See Teller-Ulam design.

^ Dr. John C. Clark,
as told to Robert Cahn, "We Were Trapped by Radioactive Fallout," The Saturday
Evening Post, July 20, 1957, pp. 17-19, 69-71.[1]